The microbiomes of five temperate soft corals declining in the Sea of Marmara

Pollution, sedimentation and physical destruction from net fishing are more imminent threats to temperate corals than thermal stress in the Sea of Marmara due to the unique conditions there. Our previous investigation into a massive die-off of benthic suspension-feeders in this inland sea revealed a knowledge gap concerning the bacterial microbiomes of affected corals. We therefore elucidated five unexplored/less-studied coral microbiomes, including those of Alcyonium acaule and Savalia savaglia, using 16S rDNA-amplicon sequencing-based profiling. Only the microbiome of Eunicella cavolini was found to be significantly different (p-value < 0.01) from those of Paramuricea clavata and Veretillum cynomorium. Host identity explained similar to 30% of the observed variation and was clearly less important than spatiotemporal factors in determining microbiome composition. All microbiomes included 12 to 19 highly persistent core members (e.g. from genera Pirellula, Synechococcus, Spirochaeta, Endozoicomonas, Halospirulina, Terasakiella, Pelagibius and Spiroplasma) plus another 16 to 42 resident bacteria (prevalence 50-75%). Endozoicomonas bacteria were however notably less abundant than previously reported in Mediterranean gorgonians; possibly due to anthropogenic stressors in the Sea of Marmara. Twelve core taxa were strongly associated with only one coral species (Spearman's rho > 0.6, p-value < 0.01), which indicate host preference. The functional roles of the core taxa are discussed.